Electrochemical carbon dioxide converter and liquid regenerator

ABSTRACT

A carbon dioxide conversion system for an environment includes a first gas-liquid contactor-separator downstream of the environment; an electrochemical conversion cell downstream of the first gas-liquid contactor-separator; and a cleaned ionic liquid storage intermediate the first gas-liquid contactor-separator and the electrochemical conversion cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/985,475, filed May 21, 2018, which claims the benefit of andpriority to U.S. patent application Ser. No. 15/896,150, filed Feb. 14,2018, and which claims the benefit of U.S. provisional application Ser.No. 62/463,921, filed Feb. 27, 2017; and U.S. patent application Ser.No. 15/896,156, filed Feb. 14, 2018 and which claims the benefit of U.S.provisional application Ser. No. 62/463,921, filed Feb. 27, 2017; all ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to carbon dioxide conversionand, more particularly, to apparatus and methods of carbon dioxideconversion employing gas-liquid contact and separation andelectrochemical cells.

Long duration human space travel, such as to Mars, will require a lifesupport system capable of recovering oxygen (O2) in high yield fromcarbon dioxide (CO2) exhaled by astronauts. The technological challengefalls only after spacecraft weight and susceptibility to radiation as ahurdle to long term habitation in space. On the International SpaceStation, CO2 is removed from the cabin by adsorption to solid sorbents.These are heavy and cannot be replaced or serviced in space. Accidentalwater exposure results in adsorbed water that is difficult to remove. Inaddition, a high regeneration temperature is required for CO2 desorptionfrom the solid sorbents for downstream reaction with hydrogen (H2) in aSabatier reactor to produce water. O2 is recovered from water viaelectrolysis. This is a very complicated system with many opportunitiesfor failure and a maximum O2 yield of 50%.

Establishing an environment for human use on the planet Mars willrequire a source for oxygen for respiration and for propulsion. Inaddition to recovering oxygen from exhaled carbon dioxide, it may beuseful to convert carbon dioxide from the Martian atmosphere intooxygen. The Martian atmosphere has an average pressure of 600 Pa (0.087psi), and an average temperature of −55 degrees C. It has a compositioncomprising 96% CO2, 1.9% argon, and 1.9% nitrogen.

As can be seen, there is a need for improved apparatus and methods toremove carbon dioxide from a supply gas and convert it into oxygen inenvironments such as deep space vehicles.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a carbon dioxide conversionsystem for an environment comprises a first gas-liquidcontactor-separator downstream of the environment; an electrochemicalconversion cell downstream of the first gas-liquid contactor-separator;and an ionic liquid storage intermediate the first gas-liquidcontactor-separator and the electrochemical conversion cell.

In a further aspect of the present invention, a carbon dioxideconversion system for an environment closed to ambient air or gascomprises a scrubber downstream of the environment; an electrochemicalconversion cell downstream of the scrubber; and an ionic liquid storageintermediate the scrubber and the electrochemical conversion cell;wherein the scrubber is configured to receive contaminated air from theenvironment, receive cleaned liquid absorbent from the ionic liquidstorage, and discharge cleaned air to the environment; and wherein thecell stack is configured to discharge oxygen to the environment.

In another aspect of the present invention, a carbon dioxide conversionsystem for an environment open to ambient air or gas comprises ascrubber downstream of the environment; an electrochemical conversioncell downstream of the scrubber; and an ionic liquid storageintermediate the scrubber and the electrochemical conversion cell;wherein the scrubber is configured to receive contaminated air from theenvironment, receive cleaned liquid absorbent from the ionic liquidstorage, and discharge at least one of Ar, N2, and CO to theenvironment; and wherein the cell stack is configured to discharge atleast one of O2, CO, and CH4 not to be returned to the environment.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a carbon dioxide conversion systemaccording to an embodiment of the present invention;

FIG. 1A is a schematic diagram of an electrochemical conversion cell ina carbon dioxide conversion system according to an embodiment of thepresent invention;

FIG. 2 is a schematic diagram of a carbon dioxide conversion systemaccording to an another embodiment of the present invention;

FIGS. 3A-3B are schematic diagrams of a carbon dioxide conversion systemaccording to a further embodiment of the present invention;

FIGS. 4A-4B are side views of a scrubber according to an embodiment ofthe present invention;

FIGS. 5A-5B are side views of a scrubber according to another embodimentof the present invention;

FIGS. 6A-6B are side views of a stripper according to an embodiment ofthe present invention;

FIGS. 7A-7B are side views of a stripper according to another embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention, since the scope of theinvention is best defined by the appended claims.

Various inventive features are described below that can each be usedindependently of one another or in combination with other features.However, any single inventive feature may not address any of theproblems discussed above or may only address one of the problemsdiscussed above. Further, one or more of the problems discussed abovemay not be fully addressed by any of the features described below.

Broadly, the present invention can be integrated into environments suchas spacecraft used in long-duration missions, specifically, spacestations and spacecraft and habitats used in and beyond low earth orbit,as the invention, particularly gas-liquid separation, operatesindependent of gravity.

The present invention generally provides a Carbon Dioxide Removal byIonic Liquid Sorbent (CDRILS) system wherein ionic liquids (ILs) can actas liquid CO2 sorbents. In contrast to solid sorbents, liquid CO2sorbents can be more easily exchanged in space, while continuous flowprocessing has the potential to afford a more robust system and a loweroverall mass. Select ILs have high capacities for CO2, chemical andelectrochemical stabilities, compatibilities with humidity in the gasstream, and negligible vapor pressures. The capacity of IL for both CO2and water allows for dual function CO2 and humidity removal, while thechemical stability of both 1-butyl-3-methylimidazolium acetate (BMIM Ac)and 1-ethyl-3-methylimidazolium acetate (EMIM Ac) makes repeatedabsorption/desorption, temperature cycling, and electrochemical cyclingpossible. The lack of vapor pressure makes ILs particularly suited tothe closed environments in space where any risk of air contamination orloss of liquids by evaporation outside the environment must be avoided.The CDRILS hollow fiber scrubber membranes provide high surface area forCO2 sorption into the ILs. A subsequent stripper membrane removes theCO2 at lower temperatures than those required for solid sorbents andregenerates the IL for recirculation to the scrubber.

The CDRILS system of the present invention can produce a CO2-loaded ILand water mixture following the scrubber. The CO2 Electrolysis in IonicLiquid (CEIL) of the current invention can electrochemically convert theCO2 and water directly to O2 and reduced carbon species such as carbonmonoxide (CO), methane (CH4), and/or other products using IL as theelectrolyte solution. The O2 and reduced carbon products are easier tostrip from the IL than CO2.

When the CDRILS and CEIL systems are used for life support, productionof O2 and CO increases the maximum possible O2 yield from CO2 from 50%to 67%, when followed by a reactor configured for carbon monoxidemethanation similar to a Sabatier reactor for carbon dioxidemethanation. The reduced carbon species or further reduced products ofthe reactor may be transformed further to recover additional valuableproducts, or they can be used as fuels.

Additionally, the present invention could be used for In-situ ResourceUtilization (ISRU) if the CO2 source is the Mars atmosphere. If thedesired target of ISRU is life support, CO2 can be electrolyzed to COand O2. If the target is fuel generation for propulsion, CO2 and watercan be electrolyzed to CO, H2, and/or methane (CH4) and O2. CO and H2would be further converted to CH4 at a second catalyst, orelectrochemically generated CH4 could be utilized directly. The O2product would be stored for use when burning the fuel.

The present invention also enables water electrolysis to H2 and O2 inthe same cell as CO2 transformation. Water electrolysis in the sameelectrolysis cell(s) could eliminate the need for the current O2generation cell stack.

The invention uses a hollow fiber-based contactor for CO2 and waterremoval from the cabin air or Mars atmosphere. The IL mixed with wateris flowed through either the lumen or the shell of the hollow fibermodule, while the cabin air or Mars atmosphere is flowed countercurrentthrough either the shell or the lumen respectively. The CO2-loaded ILand water mixture is continuously flowed through the cathode compartmentof an electrochemical conversion cell. The cathode compartment isseparated from the anion compartment by an ion exchange membrane orporous separator. The anode compartment contains either a CO2-free ILsolution or an aqueous solution. A voltage is applied between the twoelectrodes to reduce CO2 and water to reduced carbon species and H2 andhydroxide at the cathode and oxidize water or hydroxide to O2 at theanode. Additional cells can be stacked to provide the appropriatescaling for the necessary O2 generation rate.

The O2-laden liquid from the anode is then flowed to a stripper orcentrifugal separator for O2 separation, which can then be sent to thecabin or stored. The liquid is recycled to the anode compartment. Theproduct-laden liquid from the cathode is flowed to a hollow fiberstripper. The liquid is flowed through the lumen or shell, while lowpressure on the other side of the fiber strips the products and waterfrom the IL to recover the mixture required for the scrubber inlet. Theproducts are flowed to the methanation catalyst for water and methaneproduction.

US patent application entitled “Apparatus and Methods for EnhancingGas-Liquid Contact/Separation” filed Feb. 1, 2017, Ser. No. 15/422,170;US patent application entitled “Ionic Liquid CO2 Scrubber forSpacecraft” filed Feb. 1, 2017, Ser. No. 15/422,166; US patentapplication entitled “Hollow Fiber Membrane Contactor Scrubber/Stripperfor Cabin Carbon Dioxide and Humidity Control” filed Feb. 14, 2018, Ser.No. 15/896,150; and US patent application entitled “Dual Stripper withWater Sweep Gas” filed Feb. 14, 2018, Ser. No. 15/896,156, areincorporated herein by reference as though fully set forth herein.

Herein, the term “absorbent” is intended to generally include absorbentsand/or adsorbents.

“Used liquid absorbent” means “clean liquid absorbent” that has absorbedcarbon dioxide.

“Cleaned liquid absorbent” means liquid absorbent that is substantiallyfree of absorbed carbon dioxide.

“Regenerated liquid absorbent” means used liquid absorbent that hasundergone desorption of carbon dioxide.

“Cleaned gas” means gas that has an insubstantial amount of carbondioxide and/or H2O. “Cleaned gas” has a carbon dioxide and/or H2Oconcentration less than that of the gas from the environment to becleaned.

FIG. 1 is a schematic diagram of an exemplary carbon dioxide conversionsystem (i.e., closed-loop air revitalization system) 100 that may beemployed in a space-based system. A “closed-loop air revitalizationsystem” is intended to mean a system which recovers valuable resourcesfrom waste products, such as recovering valuable oxygen from wastecarbon dioxide. The system 100 may include one or more gas-liquidcontactor-separators to effectuate removal of carbon dioxide However,the system 100 is not limited to the contactor-separators describedbelow.

The carbon dioxide conversion system 100 may receive a contaminated air101 from an enclosed environment 140 suitable for human occupants, suchas a spacecraft cabin. Thus, the environment 140 can be configured to beclosed to, or sealed from, ambient air or gas. The contaminated air orgas 101 may include one or more gas/vapor contaminants such as N2, CO2,and/or H2O.

The air or gas 101 may flow into a first gas-liquid contactor-separator(i.e., scrubber) 102. In embodiments, the contaminated air 101 may,before entering the scrubber 102, be filtered for dust and particulates,via a filter 110, as well as being forced, via a fan or compressor 111,into the scrubber 102. In some embodiments, the contaminated air 101 mayexit the fan 111 and enter a cooler 113 before entering the scrubber102.

Concurrently with, or sequentially with, the scrubber 102 receiving thecontaminated air 101, cleaned liquid absorbent may be pumped, via a pump112, into the scrubber 102, from a cleaned liquid absorbent storage 105.In embodiments, the liquid absorbent may be one or more ionic liquidsdescribed below. In the scrubber 102, the cleaned ionic liquid(s) mayabsorb contaminants, such as CO2, from the contaminated air 101.

From the scrubber-separator 102, cleaned air 103 may optionally flowthrough a filter 114, to capture any leaked ionic liquid and/or producea further cleaned air that can flow back to the environment 140 to beconditioned. In embodiments, the cleaned air 103 may have a gascontaminant(s) concentration, and/or H2O concentration, lower than thatof the contaminated air 101.

Also from the scrubber-separator 102, used liquid absorbent may exit.The used liquid absorbent may flow into an electrochemical conversioncell 130 or, in embodiments, a plurality of electrochemical conversionscells. In embodiments, the plurality of cells may be in a stackconfiguration.

FIG. 1A schematically depicts an exemplary electrochemical conversioncell 130. The cell 130 may have an anode, cathode and an ionic membranetherebetween, all of which are in a bath of ionic liquid. The halfreaction at the anode that generates oxygen is either 2 H2O→O2+4 H++4 e−or 4 OH—→2 H2O+4 e−+O2, depending on the pH. Multiple reactions areproposed to occur at the cathode. These include production of hydrogen:2 H++2 e−→H2 or 2 H2O+2 e−→2 OH—+H2, and production of carbon monoxide:CO2+2H++2e−→CO+H2O or 2H2O+2 CO2+4 e−→2 CO+4 OH—. Under somecircumstances, and depending on the cathode material, methane may alsobe generated: CO2+8H++8e−→CH4+2 H2O or 6 H2O+CO2+8 e−→CH4+8 OH—.

The cell 130 can receive a used ionic liquid having CO2. The cell 130can discharge O2 to the environment 140. Concurrently with, orsequentially with, the O2 discharge, the cell 130 may discharge cleanedionic liquid to the ionic liquid storage 105. Concurrently with, orsequentially with, the O2 and cleaned ionic liquid discharge, the cell130 may discharge CO and/or H2 to a CO methanation reactor 115. The COmethanation reactor functions by reacting the carbon monoxide withhydrogen to convert it to methane and water. The water, in turn, may beelectrolyzed to generate hydrogen and oxygen, forming a closed-loop airrevitalization system.

As noted above, the electrochemical conversion cell 130 may be presentin the form of a stack of cells. Such a stack may be as described inFry, A. J., Electrochemical Processing, Organic. In Kirk-OthmerEncyclopedia of Chemical Technology, John Wiley & Sons, Inc.: 2000, FIG.10, which is incorporated herein in its entirety.

FIG. 2 is a schematic diagram of another exemplary embodiment of thepresent invention. FIG. 2 is similar to that described in relation toFIG. 1 . Accordingly, reference numbers in FIG. 2 correspond to likereference numbers in FIG. 1 .

However, in the exemplary embodiment of FIG. 2 , a carbon dioxideconversion system 200 is provided in the context of an environment 240open to ambient air or gas—such as a Martian atmosphere—as opposed tothe closed environment 140 of FIG. 1 . In this exemplary instance, theenvironment 240 is the Martian atmosphere. From the Martian atmosphere240, feed gas 201 to the system may include CO2, Ar, N2, O2, H2O, CO,and/or other gases present in the Martian atmosphere 240.

In FIG. 2 , a scrubber 202 may separate CO2 from the feed gas 201 anddischarge a cleaned gas 203 that contains less CO2 than feed gas 201 tothe Martian atmosphere 240. The discharged gas 203 may include Ar, N2,O2, CO, and/or other gases present in the Martian atmosphere 240 orproduced by the conversion cell(s) 230. An electrochemical conversioncell(s) 230 (or cell stack) may discharge CO into the Martian atmosphere240, rather than being sent to a CO methanation reactor 115 as in FIG. 1. Also, O2 discharged from the conversion cell(s) 230 can be sent to anoxygen storage unit 250 for eventual use by human occupants in a spacecraft or habitat, for example. This is in contrast to O2 being sent fromthe conversion cell 130 to the cabin 140 in FIG. 1 .

FIG. 3A is a schematic diagram of another exemplary embodiment of thepresent invention. FIG. 3A is similar to that described in relation toFIG. 2 . Accordingly, reference numbers in FIG. 3A correspond to likereference numbers in FIG. 2 .

However, in the embodiment of FIG. 3A, a cooler 313 may be intermediatea pump 312 and a scrubber 302, whereas there is no similar cooler inFIG. 2 . A heat exchanger 306 may be intermediate the scrubber 302, aclean ionic liquid storage 305, an electrochemical conversion cell(s)330 (or cell stack), and a second gas-liquid contactor-separator (i.e.,stripper) 308. In the embodiment of FIG. 2 , there is no similar heateror stripper.

Accordingly, in the embodiment of FIG. 3A, the scrubber 302 maydischarge a used liquid absorbent having CO2 and H2O, for example. Thatused liquid absorbent can be received by the heat exchanger 306 and thendischarged to the conversion cell(s) 330. Concurrently with, orsequentially with, the foregoing, the heat exchanger 306 may receivecleaned liquid absorbent from the stripper 308 and discharge the same tothe clean ionic liquid storage 305. Thus, cleaned liquid absorbent canbe pumped into the scrubber 302 where CO2 is separated from the feed gas301, and discharge cleaned gas 303 containing Ar, N2, O2, CO, and/orother gases present in the Martian atmosphere 340 or produced by theconversion cell(s) 330 may be discharged into the Martian atmosphere.

The electrochemical conversion cell(s) 330, upon receiving the ionicliquid with CO2 and H2O from the scrubber 302 and performingelectrochemical conversions, may discharge ionic liquid with CH4 to aheater 307, and the same may then flow to the stripper 308. CH4 mayexist in the discharged ionic liquid as a result of the characteristicsof the cathode and solubility of CH4 in the ionic liquid. The stripper308 may remove CH4 from the ionic liquid to thereby discharge cleanedionic liquid that can flow into the ionic liquid storage 305. The CH4removed by the stripper 308 may be discharged to a pump 317, a condenser318, and a water extractor 322 for receipt by a methane storage 361. Thesystem may also be operated without heater 307, stripper 308, and pump317, where gaseous products of the conversion cell(s) 330 cathode aredischarged to condenser 318 if solubility of CH4 in the ionic liquid islow. Stored methane may then be used as fuel in a space craft, forexample.

Concurrently with, or sequentially with, the conversion cell(s) 330discharging ionic liquid with CH4, the conversion cell(s) 330 may alsodischarge O2 through a pump 351, a condenser 352, and a water extractor353 for receipt by an oxygen storage 360. Stored oxygen may then be usedby human occupants in a space craft, for example.

From the water extractor 353, liquid water may flow to a water vaporizer323. Discharged liquid water therefrom may be received in a waterstorage 316, while discharged water vapor therefrom may flow to thestripper 308 wherein the water vapor can be used as a sweep gas.

FIG. 3B is a schematic diagram of another exemplary embodiment of thepresent invention. FIG. 3B is similar to that described in relation toFIG. 3A. Accordingly, reference numbers in FIG. 3B correspond to likereference numbers in FIG. 3A.

The embodiment of FIG. 3B is largely the same as the embodiment of FIG.3A. However, in FIG. 3B, the electrochemical conversion cell(s) 330discharges ionic liquid with CO rather than with CH4 as in FIG. 3A.Thus, the stripper 308 discharges CO rather than CH4. And the dischargedCO is received by a CO storage 371 for eventual use by human occupantsin a space craft or habitat, for example.

The invention could also be used in several other configurations.

In one embodiment, the anode compartment contains ionic liquid and/or anaqueous electrolyte solution. O2 is recovered through a gas diffusionelectrode directly as O2 in FIGS. 3A and 3B, or by passing the solutionfrom the anode compartment through a stripper, pump, compressor, andwater extractor as in methane and CO FIGS. 3A and 3B.

In this embodiment, the anode compartment does not contain an IL oraqueous solution separate from the solution in the cathode compartment.Instead, water or hydroxide passes through the ion exchange membrane orporous separator from the cathode compartment and is electrolyzed on theanode surface to O2. This removes the need for O2 separation from anelectrolyte solution. O2 is recovered as in O2 in FIGS. 3A and 3B.

In CO2 removal applications where closed-loop O2 recovery is unnecessary(e.g., aircraft, submarines), electrochemical H2 and CO fuel productioncould be targeted. The O2, H2, and CO products could be collected foruse in fuel cells.

FIGS. 4A-4B depict an exemplary embodiment of a scrubber 402 that may beemployed in the carbon dioxide conversion system 100, for example. InFIG. 4A, the scrubber 402 may include a cylindrical housing 402 a thatencloses a hollow fiber bundle 402 b. CO2-rich gas may enter the housing402 a at one end thereof and CO2-lean gas may exit at an opposite endthereof. Regenerated or cleaned liquid absorbent may enter the housing402 a at one side thereof, and used liquid absorbent with carbon dioxidemay exit the housing 402 a at an opposite side thereof. In thisembodiment, regenerated or cleaned absorbent liquid flows counter (i.e.,opposite) to the gas flow. Moreover, the counter flow causes carbondioxide to flow from the gas out of the hollow fiber bundle 402 b intothe liquid.

FIG. 4B depicts the same flows as in FIG. 4A, but in the context of asingle hollow fiber 402 c that can be part of the hollow fiber bundle402 b.

FIGS. 5A-5B depict another exemplary embodiment of a scrubber 502 thatmay be employed in the carbon dioxide conversion system 100, forexample. As in FIG. 4A, in FIG. 5A, the scrubber 502 may include acylindrical housing 502 a that encloses a hollow fiber bundle 502 b.However, CO2-rich gas may enter the housing 502 a at one side thereofand CO2-lean gas may exit at an opposite side thereof. Regenerated orcleaned absorbent liquid may enter the housing 502 a at one end thereof,and used absorbent liquid with carbon dioxide may exit the housing 502 aat an opposite end thereof. As in FIG. 4A, in this embodiment, cleanedabsorbent liquid flows counter (i.e., opposite) to the gas flow.However, the counter flow causes carbon dioxide to flow from the gasinto the liquid in the hollow fiber bundle 502 b.

FIG. 5B depicts the same flows as in FIG. 5A, but in the context of asingle hollow fiber 502 c that can be part of the hollow fiber bundle502 b.

FIGS. 6A-6B depict an exemplary embodiment of a stripper 608 that may beemployed in the carbon dioxide conversion system 300, for example. InFIG. 6A, the stripper 608 may include a cylindrical housing 608 a thatencloses a hollow fiber bundle 608 b. Sweep gas may enter the housing608 a at one end thereof and contaminants (CO2, H2O, CH4, and/or CO) mayexit at an opposite end thereof. Used absorbent liquid with contaminantsmay enter the housing 608 a at one side thereof, and regenerated orclean absorbent liquid may exit the housing 608 a at an opposite sidethereof. In this embodiment, used absorbent liquid with contaminantsflows counter (i.e., opposite) to the sweep gas flow. Moreover, thecounter flow causes contaminants to flow from the liquid into the gas inthe hollow fiber bundle 608 b.

FIG. 6B depicts the same flows as in FIG. 6A, but in the context of asingle hollow fiber 608 c that can be part of the hollow fiber bundle608 b.

FIGS. 7A-7B depict another exemplary embodiment of a stripper 708 thatmay be employed in the carbon dioxide conversion system 300, forexample. As in FIG. 6A, in FIG. 7A, the stripper 708 may include acylindrical housing 782 a that encloses a hollow fiber bundle 708 b.However, sweep gas may enter the housing 708 a at one side thereof andcontaminants may exit at an opposite side thereof. Used absorbent liquidwith contaminants may enter the housing 708 a at one end thereof, andregenerated or clean absorbent liquid may exit the housing 708 a at anopposite end thereof. As in FIG. 6A, in this embodiment, sweep gas flowscounter (i.e., opposite) to the used absorbent liquid with contaminantsflow. However, the counter flow causes contaminants to flow from theliquid out from the hollow fiber bundle 708 b into the gas.

FIG. 7B depicts the same flows as in FIG. 7A, but in the context of asingle hollow fiber 708 c that can be part of the hollow fiber bundle708 b.

According to the present invention, the liquid absorbent can meet ademanding set of criteria. The liquid can be safe and nontoxic tohumans, and may not contaminate the purified air with odors or organicvapors. It may absorb carbon dioxide at the partial pressure expectedduring the mission, and may not lose performance when simultaneouslyabsorbing water. It may also be regenerable without the use of spacevacuum, so as not to lose CO2 and water to space, and regenerablewithout using excessive temperatures or power. The liquid may be durableand last without deterioration for the life of the mission. Since theliquid also serves as the electrolyte in the electrochemical cell, itmay have appreciable electrical conductivity, and it may be stable withrespect to electrochemical oxidation or reduction within the potentialwindow for the process.

The liquid absorbent can be one or more ionic liquids. They are salts,generally comprised of an anion and organic cation, which are liquid attheir temperature of use. Because they are salts, they have effectivelyzero vapor pressure, thus eliminating odors and reducing the likelihoodof contaminating the purified air. Also, because they are salts, theyhave good electrical conductivity. They are generally nontoxic and havesufficient stability to resist deterioration. Ionic liquids generallycontain relatively large organic cations (quaternary ammonium orphosphonium compounds) and any of a variety of anions, both of which canbe tailored to obtain desired characteristics. Ionic liquids can bothphysically dissolve carbon dioxide and have specific chemicalinteractions with it. Interactions of some ionic liquids with carbondioxide and electrode surfaces favor electrochemical conversion ofcarbon dioxide into specific products. As a class, almost every ionicliquid is water soluble and hygroscopic, meaning that they will absorbmoisture from the air, but due to their negligible volatility, the watercan be removed by evaporation either by elevating the temperature orreducing the water partial pressure. Because a very large number ofionic liquids exist, and both the cation and the anion can be tailoredto obtain desired characteristics, this class of compounds hasflexibility as the liquid absorbent and electrolyte for a carbon dioxideremoval and conversion system.

Ionic liquids suitable for use in this invention comprise those withmelting points below 20° C., low vapor pressure, and with capacity forcarbon dioxide, at 30° C. and in the presence of 3.8 torr carbon dioxidepartial pressure, of >0.3 wt %. Examples of such ionic liquids include1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazoliumtrifluoracetate, 1-butyl-3-methylimidazolium acetate,tributylmethylphosphonium acetate, triethylmethylphosphonium acetateetc. These ionic liquids are hygroscopic and can absorb water as welland carbon dioxide. Therefore, the effective working fluid can, in manycases, comprise a mixture of the ionic liquids specified and water. Itmay, under some circumstances, be useful to add water to the ionicliquid before contacting with carbon dioxide. This can reduce the carbondioxide capacity but also reduce the viscosity.

1-butyl-3-methylimidazolium acetate (BMIM Ac) has a high CO2 capacityand well understood physical properties. BMIM Ac satisfies the basicrequirements for an absorbent in a manned vehicle. It is not a hazardoussubstance or mixture, and has no hazards not otherwise classified. ThepH of an aqueous solution is 6.1, and the autoignition temperature is435° C. The compound is a clear, somewhat viscous liquid, and can behandled readily. The surface tension is similar to that for a polarorganic solvent, and the density is similar to that for water. The onsetfor thermal degradation sets the upper temperature limit for processing,and is comfortably higher than the temperature needed for desorption.The viscosity for this ionic liquid is higher than that of water, butcan be reduced by raising either the temperature or water content. Innormal use, the ionic liquid absorbs both CO2 and water, and thereforethe viscosity values vary in the presence of water. Viscosity plays arole in determining mass transfer rates for CO2 adsorption anddesorption. Control of viscosity can therefore reduce the weight andvolume of the contactor-separator.

According to the present invention, a significant increase in O2recovery could be a game-changer for NASA and commercial interestspursuing travel to Mars or space habitation. Electrochemical technologymay significantly decrease the amount of O2 (and/or water) that will berequired to be brought on each mission. This saves both space on thevessel and launch weight. In addition, the present invention may requireless power than alternative CO2 conversion technologies for recovery ofthe same amount of O2.

It should be understood, of course, that the foregoing relates toexemplary embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

We claim:
 1. A carbon dioxide conversion system for an environment thatis ambient gas, comprising: a scrubber downstream of the environment; anelectrochemical conversion cell downstream of the scrubber; an ionicliquid storage intermediate the scrubber and the electrochemicalconversion cell; a stripper intermediate the electrochemical conversioncell and the ionic liquid storage, wherein: the scrubber is configuredto receive gas containing carbon dioxide from the environment, receivecleaned liquid absorbent from the ionic liquid storage, and discharge atleast one of Ar, N2, and CO to the environment, and the stripper isconfigured to (i) use water vapor as a sweep gas, (ii) discharge cleanedliquid absorbent to the ionic liquid storage, and (iii) discharge CO toa CO storage.
 2. The system of claim 1, further comprising a watervaporizer upstream of the stripper.
 3. The system of claim 1, whereinthe electrochemical conversion cell is configured to discharge O2 to anoxygen storage.
 4. The system of claim 3, further comprising: a waterextractor disposed intermediate the electrochemical conversion cell andthe oxygen storage, the water extractor configured to extract water fromthe oxygen.
 5. The system of claim 4, further comprising: a watervaporizer downstream of the water extractor and configured to discharge(i) liquid water to a water storage and (ii) water vapor to thestripper.
 6. The system of claim 1, wherein the scrubber is furtherconfigured to discharge used liquid absorbent having CO2 and H2O.
 7. Thesystem of claim 1, further comprising a heater disposed intermediate theelectrochemical conversion cell and the stripper.